Billions of people in cities around the world are exposed to dangerous air, but pollution levels vary widely – and the fast-growing cities of Asia and Africa are the worst affected.

We’ve broken down data from the World Health Organization on ultra-fine particles of less than 2.5 microns (PM2.5s) region by region. Paris’s air may have almost twice as many PM2.5s as WHO recommended levels (18 micrograms per cubic metre compared with 10µg/m³) – but Delhi’s air contains 122µg/m³, while Zabol, Iran, is the worst at 217µg/m³.

The most polluted cities in the US and Canada are dominated by California. The highest ranking goes to the small cities of Visalia and Porterville, midway between San Francisco and Los Angeles, in an agricultural valley known as “America’s salad bowl”.

PM10 pollution

Ranking the 20 worst cities in the world for pollution of PM10s – which are larger but considered slightly less dangerous than PM2.5s – puts Onitsha in Nigeria in clear last place. The fast-growing port and transit city recorded PM10 levels almost 30 times the WHO recommended level of 20µg/m³.

• The table above headed “10 worst in the United States and Canada” was amended on 7 March 2017. An earlier version included Courtenay, Canada, with a figure of 17 micrograms per cubic metre. That was the figure recorded in the World Health Organization database for the PM2.5 annual mean in Courtenay in 2013, but the WHO now says it was incorrect and that the correct 2013 figure is 11 micrograms per cubic metre.

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